The invention relates to ink jet printing apparatus as used for applying characters, for instance letters or figures, on packages or other objects to be marked, which are advanced along a path through or past a marking station by suitable advancing means. More particularly, the invention relates to an ink jet printing apparatus of the type comprising a plurality of ink jet nozzles, means for supporting said nozzles in a row in which the axes of the nozzles define a common plane, a solenoid valve (that is, an electromagnet operated valve) for controlling the supply of pressurized ink to each of said nozzles, a control circuit for each of said solenoid valves, and a pattern generator having a plurality of output leads connected to said control circuits, said pattern generator being arranged successively to select predetermined output leads or combinations of output leads and to cause an operating pulse to be transmitted through said selected output leads and control circuits to the respective solenoid valves controlled thereby.
The opening of the valve caused by said pulse results in the ejection of a small volume of ink which when hitting the surface to be marked forms a point or spot of ink on the same. When a combination of two or more output leads is selected, resulting in the operation of two or more valves at a time, the resulting two or more spots on the surface of the object form a row extending transversally of the direction of advancement of the object. The characters to be applied may consist of a single spot, or a single row of spots, or, more frequently, of several rows of spots forming one of a plurality of predetermined patterns determined by the programming of the pattern generator, the spacing between successive rows being also determined by the pattern generator.
It is a principal object of the invention to improve apparatus of the kind above set out by the provision of means for effecting a quick change-over between two (or more) character sizes, without any exchange or mechanical adjustment of the nozzles.